A yarn wetting device and a two-for-one twisting spindle provided with a yarn wetting device is described in, for example, U.S. Pat. No. 3,864,901, issued Feb. 11, 1975 and in the publication "Chemiefasern/Textillindustrie" 27/79 (1977), 1018-1021.
An essential part of such wetting device is a wetting body in the form of a porous body having a capillary action and disposed to absorb wetting agent or lubricant directly from the reservoir and/or additionally by way of a wick system. The yarn is drawn over the wetting body and thereby receives lubricant or wetting agent. The wetting agent or lubricant diffuses into and through the wetting body as a result of capillary forces and forms a thin film of liquid or lubricant on that surface of the wetting body which faces the yarn. The equilibrium in the wetting body capillary system is changed when the yarn removes this film, and fresh lubricant or wetting agent is conveyed to the surface. This operation takes place continuously. As a result of this physical principle, the lubricating unit cannot drip in the event of the breakage of the yarn or stoppage of the machine.
The conveying of lubricant by capillary forces makes the lubricating system dependent upon various factors, such as the chemical constitution and viscosity of the lubricant or wetting agent, average size of pore and distribution of the size of pores in the wetting body, and the level in the reservoir.